The present invention pertains to connecting integrated circuits (i.e., "chips") to a substrate for thick film hybrid circuits. It particularly pertains to C4 bump pads for connecting integrated circuits; and, more particularly, the invention pertains to a technique for easier and more reliable laying down solder of bumps on substrate conductors.
C4 is an advanced microelectronic integrated circuit packaging and connection technology. "C4" stands for "Controlled Collapse Chip Connection." C4 relates to the interrelation of "solder bumps" and "flip chips".
The basic idea of C4 is to connect integrated circuits, integrated circuit packages, or other such units to conductor strips or other like connections on substrates or integrated circuits by means of solder bumps partially flattened between two surfaces of the units. The tiny C4 bumps of electrically conductive solder may be arrayed on the surface of one unit and are pressed against metal pads on another unit to make one electrical connection at each solder ball. C4 allows all the connections to be simultaneously made in one step during a reflow operation.
A major application of C4 is in joining semiconductor microchips (integrated circuits) to integrated circuit packages or substrates. The integrated circuits typically are made in repeating rectangular arrays on a mono-crystalline disc of silicon, several inches (or centimeters) across, called a "wafer." Many integrated circuits are formed on each wafer before the integrated circuits are separated into single units. C4 bumps are placed on the metal terminals on the integrated circuits while they are still part of the wafer. Wafers are made as large as possible so as to make the most integrated circuits at once, and the integrated circuits are made as small as possible. The best C4 fabrication system is one that makes very small, closely-spaced solder bumps each precisely placed over a large area. Then the respective integrated circuits may be cut from and isolated from the larger wafer.
C4 is commercially important because it allows a very high density of electrical interconnections. C4 can be used with perimeter connection techniques, similar to tape automated bonding (TAB), but C4 connections can also be arrayed over surfaces. When an area is covered, the number of possible connections for a given size of a unit is roughly squared. C4 bumps are typically about a hundred microns in diameter and the connector density is on the order of several thousand per square inch (6.45 square centimeters).
C4 solder bumps must be and are rugged. A computer or other electronic devices, with dozens of integrated circuits and thousands of C4 solder bump connections, easily can be rendered nonfunctional if only one of the C4 bump connection fails.
One method of fabricating the C4 solder bumps on integrated circuits is by evaporation or vacuum deposition. In this process, terminal metals are first evaporated in a vacuum chamber and these metals are deposited on a wafer through a metal mask. This is followed by evaporation of solder metal, which is deposited through the metal mask on top of the terminal metals. The terminal metals form the ball limiting metallurgy (BLM) and the solder metals constitute the solder bumps.
As shown in FIG. 1, a C4 solder ball 10 (shown as a cross section) is deposited on a metal conductive pad or strip 12 situated on a surface 16 of wafer or substrate 14. The substrate might be polyimide, silicon (Si), quartz (SiO.sub.2), ceramic, beryllium oxide (BeO) or other material. In place of the substrate may be a printed circuit board of appropriate material. Ball 10, approximately ranging between 100 and 200 microns in diameter, is preferably composed of about 97 percent lead (Pb) and 3 percent tin (Sn). Pad or strip 12 may have a composition of one part palladium and two parts silver. Other compositions may be used. The solder may be electro-deposited in the form of a cylinder, and subsequently reflowed into bumps.
While adhesion pulls the molten solder drop outward, an opposing force of cohesion pulls it inward. Cohesion is the molecular attraction of a substance to itself. In a liquid it causes surface tension. The surface tension in the molten solder drop tries to reduce the drop's surface area and make it spherical. If the solder does not wet the conductive pad, layer or strip 12 at all, the solder will ball up into an almost-perfect sphere and barely touch the surface. The force of gravity is negligible for such small droplets because the surface area-to-volume ratio is inversely related to diameter. If the two forces of adhesion and cohesion are equal, the drop will take on the hemispherical shape shown by soap bubbles on a soapy table, where surface tension pulls equally in and out.
FIG. 2 is a schematic partial cross section of a flip chip 18 attached to the top of a chip carrier substrate 14. A grid array of C4 solder joints 10 mechanically and electrically connect between an array of metal contacts 20 on flip-chip 18 and a mirror image array of metal contacts 12 on top side 16 of substrate 14.
The related art design involves performing a flip chip 18 attachment on a thick film hybrid circuit of substrate 14 using the C4 solder bump process. This process requires the hybrid circuit design to include C4 solder pads 12 that match the flip chip die 18 solder pads 20. Solder pads 12 and 20 must maintain specific dimensions (i.e., 8 to 11 mils or 200 to 280 microns in diameter) in order for the C4 process to be properly accomplished. Solder pads that are too large may result in solder from the pad causing a short to an unpassivated edge on die 18. Solder pads that are too small may cause die placement problems as well as reliability problems for the resultant circuit. C4 pads 12 consist of a thick film conductor material 11 that has been passivated by a dielectric or encapsulant 22 with an opening in it to create a pad 12. The screen printed encapsulant material is designed to flow and close pin holes or slits caused by the printing processes. This property of the encapsulant material inhibits the forming of small C4 pad openings over the conductor material. To minimize the problems with the encapsulant flow, the C4 pads are located on the circuit by creating a series of square pads out of conductor and glass encapsulant (see FIG. 3).
FIG. 3 reveals the related art C4 solder pad design. By applying an encapsulant 22 with a rectangular opening 24, the ends are allowed to flow while the area that creates C4 pads 12 is monitored. This reduces the problem of encapsulant flowing, but does not eliminate it. These square pads 12 are designed to be aligned with octagonal pads 20 on flip chip 18. When solder is applied to pads 20 on flip chip 18, they take on a round shape. The solder on the thick film circuit has a square shape at the base and a round shape at the top. Proper registration of the bumps relative to one another, as well as size, is critical so that the die can be positioned properly during the C4 bump reflow process.